Superconductor Electronics Fabrication Process with MoNx Kinetic Inductors and Self-Shunted Josephson Junctions

Recent progress in superconductor electronics fabrication has enabled single-flux-quantum (SFQ) digital circuits with close to one million Josephson junctions (JJs) on 1-cm
²
chips. Increasing the integration scale further is challenging because of the large area of SFQ logic cells, mainly determined by the area of resistively shunted Nb/AlO
_x
-Al/Nb JJs and geometrical inductors utilizing multiple layers of Nb. To overcome these challenges, we are developing a fabrication process with self-shunted high-J
_c
JJs and compact thin-film MoN
_x
kinetic inductors instead of geometrical inductors. We present fabrication details and properties of MoN
_x
films with a wide range of T
_c
, including residual stress, electrical resistivity, critical current, and magnetic field penetration depth λ
₀
. As kinetic inductors, we implemented Mo
₂
N films with Tc about 8 K, λ
₀
about 0.51 μm, and inductance adjustable in the range from 2 to 8 pH/sq. We also present data on fabrication and electrical characterization of Nb-based self-shunted JJs with AlOx tunnel barriers and Jc = 0.6 mA/μm
²
, and with 10-nm thick Si
_1-x
Nbx barriers, withx from 0.03 to 0.15, fabricated on 200-mm wafers by co-sputtering. We demonstrate that the electron transport mechanism in Si
_1-x
Nbx barriers at x <; 0.08 is inelastic resonant tunneling via chains of multiple localized states. At largerx, their Josephson characteristics are strongly dependent onx and residual stress in Nb electrodes, and in general are inferior to AlO
_x
tunnel barriers.